The present invention relates to compression springs.
Circular compression springs can be used in all kinds of machinery and have a number of uses in the aerospace and automotive industries for vehicle suspension systems and various other mechanical applications. Specifically, the automotive industry recently has been attempting to reduce vehicle weight in order to lower costs and provide decreased fuel consumption. The substitution of lightweight materials for heavy metallic materials commonly used in automobile parts is one method currently envisioned to decrease vehicle weight.
Metal circular compression springs are also used within multitudes of machines such as automobiles, trains, trucks, busses, etc. Fatigue failure commonly occurs in parts which are subjected to continually varying stresses, for example, circular multi-wave compression springs. However, the vibratory capacity and endurance of the metal springs limit the length of time and frequency during which these machines can be operated without replacement of the metal springs due to fatigue failure.
A metal multi-wave compression spring of the type described is disclosed in U.S. Pat. No. 4,901,987. Wave springs by their very nature, including those of the type disclosed in U.S. Pat. No. 4,901,987, are prone to fatigue failure. The springs are flexed repeatedly and the cross-section of each turn of the spring is generally thin and the spring is stressed very often to the highest possible limit. Wave springs are normally made of hardened and tempered metal in which the metal is deformed plastically to create the waves. This deformation induces residual compressive and tensile stresses in different surfaces of the wave. After loading, the residual stresses increase the effective applied stresses on the spring. Apart from the fatigue life characteristics of the spring, the load resulting from the combined residual and applied stresses may exceed the yield strength of the material and result in catastrophic failure of the spring.
Another problem associated with metal wave compression springs is the method typically used to form the spring. In order to produce multi-wave compression springs, a metal filament is typically edge wound into the circular configuration of the spring. As a result of the material twist resulting from edge winding, the contact area between the crests and troughs on adjacent turns of the multi-wave spring is not between parallel surfaces but rather the turns meet at a point. More particularly, the steel or metal multi-wave compression springs being edge wound are not uniformly planar in a radial direction outward from a longitudinal axis of the spring. As a result, only the outer edges of each crest and trough contact at a point. The point loading at each adjacent crest and trough establishes a large point loading contact stress which wears on the outer edge of the spring. The point contact results in rubbing between the adjacent crests and troughs during repeated and successive loading of the spring. The friction created at the point contact between each crest and trough results in galling on the surface of the spring filament. The point rubbing or galling reduces the fatigue life and creates a sharp edge. The sharp edge adds a large stress concentration factor further deteriorating the contact area and ultimately creating a crack initiation site in the spring which potentially leads to catastrophic failure.
Another problem associated with metallic multi-wave compression springs is the non-linearity of the spring response. The effects of the point contact between the adjacent crests and troughs of the metal spring in combination with the production methods typically used for such springs results in a linear spring rate over a very narrow deflection range or a non-linear spring response over a larger deflection range. A "linear" spring response, spring constant or spring rate as referred to herein means that a constant incremental applied force results in a constant incremental deflection of the spring.
A further problem associated with metal multi-wave springs is hysteresis. Hysteresis is the difference in a plot of the force/deflection of a spring as it is loaded compared to when the load is being removed. The difference between the force/deflection plots of the loading and unloading of a metal multi-wave spring is significant, A large hysteresis results in inconsistent spring performance; especially at high frequency loading and unloading conditions.
It has long been known to use composites materials as a subistitute for metallic leaf springs and metallic coil springs. The use of composite materials provides a low weight substitute for metal springs. However, the benefits of composite materials have not been fully realized for use in multi-wave compression springs due in part to the characteristics of the waves and the forming requirements associated with the waves. Forming the waves into the turns of a cured composite ribbon or coil spring is impractical and would result in the cracking, breaking, or destruction of the fibers and/or resin matrix of the composite material. Moreover, even if the cured composite ribbon could be edge wound as is done with metal ribbons, the crests and troughs would still be canted resulting in point contact and all of the disadvantages associated therewith.
Furthermore, by their very nature multi-wave springs have a significant amount of contact and/or rubbing at the wave crest and trough interfaces. Multi-wave springs should be capable of being used in a wide variety of environments and conditions, including industrial and automotive applications which can be very non-sterile, dirty and have many particulate contaminants. These dirt particles and other contaminants could interfer with the wave crest and trough interfaces and cut, degrade or otherwise damage the composite materials, particularly the fibers.
It has therefore been an objective of this invention to provide an improved, lightweight circular compression spring.
It has been a further objective of this invention to provide a lightweight circular multi-wave compression spring that avoids the fatigue and performance characteristics associated with metal multiwave compression springs.
It has been a still further objective of this invention to provide such a spring which offers adequate strength and provides a longer useful life.